RE: iS145 vs. FT148
What is your husband's "next size down" rig, and do you have some smaller fins for the FT-148.
I'm smaller (75 Kg) and I would sort of agree that the FT-148 feels "big" but I've had some sessions on it with the stock fin (not a 70 when I want to get going the earliest) and 8.5-9.5 rigs and these changes made the board feel qiute a bit smaller
Does he use an adjustable outhaul on the 11.0 m2 Retro?
With an adjustable outhaul he would be able to "tune the power" in the big Retro so that as sonn as it's feeling "big" he can take some power out of it.
This would be the least expensive way to find out if it's the Retro, or the FT-148 that feeling too big too quickly.
Using a 10.0m2 or 9.5m2 rig would not affect early planing by more than a knot. At your husbands weight he's right in the range to make a slightly small sail plane just as early with good technique.
His ability to get the FT-148 planing in =>7 knots tells me he already has very good technique.
If he can borrow or demo a 10.0 m2/9.5m2 Retro on alight wind day that would be the best way to find out what the differences are.
Also, where does he run his mast foot on the 11.0 m2?
At the front of the track, in the middle, or near the rear?
Changing the position of the mast foot can really affect how the F-Types feel and it may be that he needs to bring the mast foot back when he feels the board is "too big" or if he already has it back maybe move it forward a little to "settle" the board.
Hope this helps,